The role of mRNA competition in regulating translation. I. Demonstration of competition in vivo.

SC-1 cells infected with reovirus continue to synthesize host proteins at a nearly normal rate for at least 12 h postinfection. The relative translational efficiencies of reovirus and host mRNAs were determined at various times during this interval. Analysis was performed by determining the sensitivity of individual mRNA translation rates to inhibition by low doses of cycloheximide, by measuring polysome sizes, and by quantitatively comparing mRNA concentration with protein synthesis rates. Treatment of infected cells with low doses of cycloheximide actually stimulates the synthesis of some of the major reovirus protein, whereas host protein synthesis is inhibited. An analysis of the polysomes encoding two reovirus proteins (microNS and sigma 3; synthesis of both is stimulated by cycloheximide) shows that these proteins are synthesized on polysomes smaller than those producing host proteins of comparable size. Reovirus mRNAs accumulate to unusually high levels during the first 8 h of infection, accounting for 30% to 45% by weight of the total mRNA in the cell. We interpret these results as indicating that viral and host mRNAs must compete for a message-discriminatory component prior to their binding to the 40 S ribosomal complex and that this component is limiting in reovirus-infected cells. Moreover, a hierarchy must exist among mRNAs in terms of their affinity for this component, host mRNAs in general having greater affinities than reovirus mRNAs.